PH-induced azo-keto and azo-enol tautomerism for 6-(3-methoxypropylamino)pyridin-2-one based thiophene azo dyes

Abstract

The synthetic strategy of combining functional group transformation (FGT) and post-modification (PM) is proposed to prepare two new types of 2-amino-3-cyano-4-chloro-5-formylthiophene based heterocyclic azo dyes with mono and double 3-methoxypropan-1-amine substituted pyridine components, where the terminal aldehyde radical of the 2-formylthiophene moiety is extended into imine, acetal or α,β-unsaturated cyanoacetic ester group. The evaluation of pH stability of the mono FGT products indicates that the basic stability is depended upon the π-conjugated system of the diazo components by comparing our synthesized thiophene and azobenzene based dyes, in which better electron delocalization may facilitate the proton transfer within molecule resulting in poor stability towards base. Furthermore, the intra-ring azo-keto ⇌ azo-enol tautomerism in the coupling components is suggested to explain the appearance of five isoabsorptive points in their pH-titration experiments. In contrast, all the N2,N6-bis(3-methoxypropyl)pyridine-2,6-diamine based azo dyes exhibit extremely high pH stability no matter what diazo components are used, because no proton accepting sites are present after the double FGT. It is also found that the synthesized thiophene heterocyclic dyes exhibit a large color gamut after PM. For example, the maximum absorption wavelength (λmax) is significantly red-shifted from 496 nm in the acetal compound to 590 nm in the cyanoacetic ester terminated dye.